This application relates in general to measurement and sensing of low power signals. More particularly, the invention relates to the sensing, amplification and measurement of a low power, light-based signal.
FIG. 1 illustrates a circuit 100 of the prior art for amplifying a signal from a photo diode 130. The circuit of FIG. 1 includes the photo diode 130 connected across the inputs of an operational amplifier 120. The positive input of the op amp 120 is tied to ground. A resistive load R 150 is coupled between the negative terminal and the out signal 110 of the op amp 120.
Notably, the feedback resistor R 150 has inherent thermal noise that can sometimes exceed the actual signal from the photo diode 130. The output from a resistive feedback amplifier such as circuit 100 is given in equation (1) below:
Vout=xe2x88x92i Rxe2x80x83xe2x80x831
where Vout is in volts, i is the input signal in amperes from a signal source (such as photo diode 130) and R is the feedback resistance (such as the resistor R 150) in ohms.
A component with resistance generates thermal noise with the following RMS values:
xe2x80x83VRMS noise={square root over (4kTBR+L )}xe2x80x83xe2x80x832
IRMS noise={square root over (4kTB/R+L )}xe2x80x83xe2x80x833
where VRMSnoise is in volts and IRMSnoise is in amperes and where k=1.38xc3x9710xe2x88x9223 J/xc2x0K (Boltzmann""s constant), T is the absolute temperature in xc2x0K, B is the bandwidth in Hz and R is the resistance in ohms.
Therefore, when an application requires the amplification of a very low signal from a photo diode, the prior art resistive feedback amplifier 100 sometimes proves unuseful due to excessive noise, for example.
FIG. 2 presents a circuit 200 of the art, designed to avoid this thermal noise problem. In FIG. 2, the photo diode 130 remains coupled across the inputs of the op amp 120. In place of the resistive element R 150, a capacitor 220, coupled between the negative input and the output 210 of the op amp 120, serves as the feedback element. The source of a field-effect transistor (FET) 230 is coupled to the output 210 of the op amp 120 while the drain is coupled to the negative input of the op amp 120. The gate of the FET 230 serves as a Reset signal 240.
The use of the capacitor 220 as the feedback element eliminates the noise problem of the circuit 100.
The output from an integrator such as the circuit 200 is given in equation (4) below:
Vout=xe2x88x92i t/Cxe2x80x83xe2x80x834
where i is the input signal from a signal source (such as photo diode 130) in amperes, t is the time from reset to reading in seconds and C is the feedback capacitance (of capacitor 220, for example) in farads.
FIG. 3 illustrates the timing of the operation of the circuit 200 of FIG. 2. A control circuit (not shown) typically resets the integrator 200 (by means of the Reset signal 240) at twice the rate of the signal bandwidth. Just prior to each of these resets, the control circuit reads the out signal 210 and extracts the true signal.
The use of the semiconductor switch 230, however, creates its own problems in the circuit 200. The charge transfer itself from the Reset signal 240 during the resetting of the integrator 200 induces noise. To avoid this problem, the control circuit reads the out signal 210 right after releasing the reset switch 240. The control circuit then subtracts this reading from the final reading.
The noise of the photo diode 130 and op amp 120 nonetheless affect the two-reading scheme used with the circuit 200 up to the bandwidth of the system. The system bandwidth has to be much higher than the signal bandwidth in order not to distort the integration curves.
Accordingly, there is a need for a circuit for an improved detector of low levels of light without the thermal noise and other problems described above. These and other goals of the invention will be readily apparent to one of ordinary skill in the art on the reading of the background above and the invention description below.
Herein is disclosed a method and apparatus for measuring very low power signals such as low power light signals, including integrating a signal from a signal source such as a photo diode, an avalanche photo diode, a photomultiplier tube or the like, digitally sampling the integrator output multiple times during each integration period, fitting a curve to the multiple digitized readings to calculate the integration slope for each integration period and determining the original signal from the calculated integration slope.
According to an aspect of the invention, an apparatus for use in measuring low power signals is provided, the apparatus comprising: an integrator, wherein the integrator receives an original low power signal from a signal source and integrates the signal over multiple integration periods; an analog-to-digital converter having an analog input coupled to an output of the integrator, wherein the converter digitally samples the integrator output more than two times during each integration period to obtain multiple digital samples; and a processor coupled to a digital output of the analog-to-digital converter, wherein the processor determines the original low power signal from the multiple digital samples.
According to another aspect of the invention, an apparatus for use in measuring low power light-based signals in a detection region in a first one of at least two intersecting microchannels is provided, the apparatus comprising: a photo diode located proximal the detection region which detects a low power light-based signal in the detection region and outputs a photo diode signal; an integrator having an input coupled to an output of the photo diode; wherein the integrator receives and integrates the photo diode signal over multiple integration periods; a low pass filter having an input coupled to an output of the integrator, wherein the low pass filter operates to filter out frequencies above a predetermined level in the integrator output signal; an analog-to-digital converter having an analog input coupled to an output of the low pass filter, wherein the converter digitally samples the filtered integrator output signal more than two times during each integration period to obtain multiple digital samples; and a processor coupled to a digital output of the analog-to-digital converter, wherein the processor calculates the integration slope for each integration period using the multiple digital samples, and wherein the processor determines. the original low power signal from the calculated integration slopes.
According to yet another aspect of the invention, a method is provided for measuring low power signals, the method comprising the steps of: receiving an original signal from a signal source; integrating over multiple integration periods the original signal with an integrator to produce an integrator output signal; digitally sampling the integrator output signal more than two times during each integration period with an analog-to-digital converter coupled to the integrator to obtain multiple digital samples; and determining the original signal from the multiple digital samples.
According to a further aspect of the invention, a method is provided for measuring low power light-based signals in a detection region in a first one of at least two intersecting microchannels, the method comprising the steps of: locating a photo diode proximal the detection region, wherein the photo diode detects an original low power light-based signal in the detection region and outputs a photo diode signal; integrating the photo diode signal over multiple integration periods to produce an integrator output signal using an integrator having an input coupled to an output of the photo diode; filtering out frequencies above a predetermined level in the integrator output signal using a low pass filter having an input coupled to an output of the integrator; digitally sampling the filtered integrator output signal more than two times during each integration period with an analog-to-digital converter having an analog input coupled to an output of the low pass filter to obtain multiple digital samples; calculating the integration slope for each integration period using the multiple digital samples; and determining the original low power signal from the calculated integration slopes.
According to yet a further aspect of the invention, a system is provided for measuring low power signals, the system comprising: means for detecting an original low power signal; means for integrating the original low power signal over multiple integration intervals to produce an integration output signal; digital sampling means for digitally sampling the integration output signal more than two times during each integration interval to obtain multiple digital samples; and a processor coupled to the digital sampling means, the processor including: means for calculating the integration slope for each integration interval using the multiple digital samples; and means for determining the original low power signal from the calculated integration slopes.
Reference to the remaining portions of the specification, including the drawings and claims, will realize other features and advantages of the present invention. Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with respect to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements.